The present invention relates to a laser lap welding method for a galvanized steel sheet. More specifically, the present invention relates to a method for processing an end portion of a welding section in a laser welding method for galvanized steel sheets lapped one over the other with no gap.
In a wide variety of industries such as the automobile industry, galvanized steel sheets are commonly used because they are high in specific strength and low in cost as well as excellent in corrosion resistance. In particular, in the automobile industry, etc., where steel sheets having large areas are used, there have been attempts to introduce laser beam welding capable of higher speed processing than spot welding and the like in order to weld together a number of galvanized steel sheets lapped one over another.
However, the melting point (approximately 420° C.) and boiling point (907° C.) of zinc are much lower than the melting point (approximately 1535° C.) of iron. Accordingly, merely lapping galvanized steel sheets followed by laser irradiation results in the formation of weld defects such as pits, porosities, and worm holes due to a phenomenon in which zinc evaporated from each galvanized layer blows away molten metal therearound or remains in the molten metal as bubbles. For this reason, as described in JP 60-210386 A, JP 61-74793 A, or JP2007-38269A, countermeasures have been developed such as providing a gap for venting zinc vapor between galvanized steel sheets to be welded together by laser lap welding, using a spacer, a difference in level, or the like. However, such a method requires much time and effort, impairing the merits of introducing laser beam welding.
Then, WO 2010/005025 A1 discloses a laser lap welding method in which a laser beam is emitted with higher power density and at higher speed than usual to thereby temporarily form an elongated hole 20 (keyhole) in a molten pool 2 behind a laser irradiation position (10) as shown in FIGS. 1(a) and 1(b), and welding (3) is performed while venting metal vapor 23 through the hole 20 toward a laser irradiation source. This method eliminates the need for an additional preparing process as described above, and enables laser beam welding immediately after galvanized steel sheets are directly lapped one over the other. Thus, a large number of galvanized steel sheets having large areas can be efficiently welded together.